LXR linux/Documentation/arm/Booting

   1                        Booting ARM Linux
   2                        =================
   3
   4Author: Russell King
   5Date  : 18 May 2002
   6
   7The following documentation is relevant to 2.4.18-rmk6 and beyond.
   8
   9In order to boot ARM Linux, you require a boot loader, which is a small
  10program that runs before the main kernel.  The boot loader is expected
  11to initialise various devices, and eventually call the Linux kernel,
  12passing information to the kernel.
  13
  14Essentially, the boot loader should provide (as a minimum) the
  15following:
  16
  171. Setup and initialise the RAM.
  182. Initialise one serial port.
  193. Detect the machine type.
  204. Setup the kernel tagged list.
  215. Call the kernel image.
  22
  23
  241. Setup and initialise RAM
  25---------------------------
  26
  27Existing boot loaders:          MANDATORY
  28New boot loaders:               MANDATORY
  29
  30The boot loader is expected to find and initialise all RAM that the
  31kernel will use for volatile data storage in the system.  It performs
  32this in a machine dependent manner.  (It may use internal algorithms
  33to automatically locate and size all RAM, or it may use knowledge of
  34the RAM in the machine, or any other method the boot loader designer
  35sees fit.)
  36
  37
  382. Initialise one serial port
  39-----------------------------
  40
  41Existing boot loaders:          OPTIONAL, RECOMMENDED
  42New boot loaders:               OPTIONAL, RECOMMENDED
  43
  44The boot loader should initialise and enable one serial port on the
  45target.  This allows the kernel serial driver to automatically detect
  46which serial port it should use for the kernel console (generally
  47used for debugging purposes, or communication with the target.)
  48
  49As an alternative, the boot loader can pass the relevant 'console='
  50option to the kernel via the tagged lists specifying the port, and
  51serial format options as described in
  52
  53       Documentation/kernel-parameters.txt.
  54
  55
  563. Detect the machine type
  57--------------------------
  58
  59Existing boot loaders:          OPTIONAL
  60New boot loaders:               MANDATORY
  61
  62The boot loader should detect the machine type its running on by some
  63method.  Whether this is a hard coded value or some algorithm that
  64looks at the connected hardware is beyond the scope of this document.
  65The boot loader must ultimately be able to provide a MACH_TYPE_xxx
  66value to the kernel. (see linux/arch/arm/tools/mach-types).
  67
  68
  694. Setup the kernel tagged list
  70-------------------------------
  71
  72Existing boot loaders:          OPTIONAL, HIGHLY RECOMMENDED
  73New boot loaders:               MANDATORY
  74
  75The boot loader must create and initialise the kernel tagged list.
  76A valid tagged list starts with ATAG_CORE and ends with ATAG_NONE.
  77The ATAG_CORE tag may or may not be empty.  An empty ATAG_CORE tag
  78has the size field set to '2' (0x00000002).  The ATAG_NONE must set
  79the size field to zero.
  80
  81Any number of tags can be placed in the list.  It is undefined
  82whether a repeated tag appends to the information carried by the
  83previous tag, or whether it replaces the information in its
  84entirety; some tags behave as the former, others the latter.
  85
  86The boot loader must pass at a minimum the size and location of
  87the system memory, and root filesystem location.  Therefore, the
  88minimum tagged list should look:
  89
  90        +-----------+
  91base -> | ATAG_CORE |  |
  92        +-----------+  |
  93        | ATAG_MEM  |  | increasing address
  94        +-----------+  |
  95        | ATAG_NONE |  |
  96        +-----------+  v
  97
  98The tagged list should be stored in system RAM.
  99
 100The tagged list must be placed in a region of memory where neither
 101the kernel decompressor nor initrd 'bootp' program will overwrite
 102it.  The recommended placement is in the first 16KiB of RAM.
 103
 1045. Calling the kernel image
 105---------------------------
 106
 107Existing boot loaders:          MANDATORY
 108New boot loaders:               MANDATORY
 109
 110There are two options for calling the kernel zImage.  If the zImage
 111is stored in flash, and is linked correctly to be run from flash,
 112then it is legal for the boot loader to call the zImage in flash
 113directly.
 114
 115The zImage may also be placed in system RAM (at any location) and
 116called there.  Note that the kernel uses 16K of RAM below the image
 117to store page tables.  The recommended placement is 32KiB into RAM.
 118
 119In either case, the following conditions must be met:
 120
 121- Quiesce all DMA capable devices so that memory does not get
 122  corrupted by bogus network packets or disk data. This will save
 123  you many hours of debug.
 124
 125- CPU register settings
 126  r0 = 0,
 127  r1 = machine type number discovered in (3) above.
 128  r2 = physical address of tagged list in system RAM.
 129
 130- CPU mode
 131  All forms of interrupts must be disabled (IRQs and FIQs)
 132  The CPU must be in SVC mode.  (A special exception exists for Angel)
 133
 134- Caches, MMUs
 135  The MMU must be off.
 136  Instruction cache may be on or off.
 137  Data cache must be off.
 138
 139- The boot loader is expected to call the kernel image by jumping
 140  directly to the first instruction of the kernel image.
 141
 142